Ablation device with optimized input power profile and method of using the same

ABSTRACT

Ablation device including a probe structure 10 having a proximal end 12 and a distal end 14. Probe structure 10 includes a tubular first catheter 16, a tubular second catheter 18 surrounding the first catheter and a tubular guide catheter extending within the first catheter 16. The first catheter 16 carries a cylindrical ultrasonic transducer 20 adjacent its distal end. The transducer 20 is connected to a source of electrical excitation. The ultrasonic waves emitted by the transducer 20 are directed at the heart wall tissue. Once the tissue reaches the target temperature, the electrical excitation is turned on and off to maintain the tissue at the largest temperature. Alternatively, the transducer 20 is subjected to continuous excitation at one power level and upon the tissue reaching the target temperature, the power level of the continuous excitation is switched to a second lower power level.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional ApplicationNo. 60/802,243, filed May 19, 2006, the disclosure of which is herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to medical procedures such as ablation andto apparatus and method for carrying out such procedures.

BACKGROUND OF THE INVENTION

Ultrasonic heating such as high intensity focused ultrasound (HIFU) isutilized for certain therapeutic applications. As disclosed in commonlyassigned International Application PCT/US98/1062, published asInternational Publication WO/98/52465 the disclosure of which is herebyincorporated by reference herein, HIFU heating typically is conductedusing an ultrasonic emitter having an array of transducers. Thetransducers are actuated with a drive signal so as to emit ultrasonicwaves. The relative phasing of the waves is controlled by the physicalconfiguration of the array and the phasing of the drive signal. Thesefactors are selected so that the ultrasonic waves tend to reinforce oneanother constructively at a focal location. Tissue at the focal locationis heated to a greater extent than tissue at other locations. Asdescribed, for example in commonly assigned U.S. Pat. No. 6,461,314 andin commonly assigned U.S. Pat. No. 6,492,614, the disclosures of whichare also incorporated by reference herein, HIFU may be applied bytransducer arrays such as arrays of polymeric piezoelectric transducers.These arrays can be mounted on a probe such as a catheter which can beintroduced into the body, for example, as in a cavernous internal organor within the vascular system to perform cardiac ablation.

Contraction or “beating” of the heart is controlled by electricalimpulses generated at nodes within the heart and transmitted alongconductive pathways extending within the wall of the heart. Certaindiseases of the heart known as cardiac arrhythmias involve abnormalgeneration or conduction of the electrical impulses. One such arrhythmiais atrial fibrillation or “AF.” Certain cardiac arrhythmias can betreated by deliberately damaging the tissue of the cardiac wall along apath crossing a route of abnormal conduction. This results in formationof a scar extending along the path where tissue damage occurred. Thescar blocks conduction of the electrical impulses. Such a scar can becreated by conventional surgery, but this entails all of the risks andexpense associated with cardiac surgery. Alternatively, the scar may bemade by application of energy such as heat, radio frequency energy orultra sonic energy to the tissue that is to be scarred. Scarring thetissue by application of energy is referred to as cardiac ablation.

Commonly assigned U.S. Pat. No. 6,635,054, the disclosure of which isincorporated by reference herein in its entirety discloses thermaltreatment methods and apparatus. The disclosed apparatus includescollapsible ultrasonic reflector. The reflector incorporates agas-filled reflector balloon, a liquid-filled structural balloon and anultrasonic transducer disposed within the structural balloon. Acousticenergy emitted by the transducer is reflected by a highly reflectiveinterface between the balloons and focused into an annular focal regionto ablate the cardiac tissue.

Commonly assigned U.S. Patent Application Publication No. US2004/0176757, the disclosure of which is incorporated by referenceherein in its entirety, discloses cardiac ablation devices. Thedisclosed devices are steerable and can be moved between a normaldisposition, in which the ablation region lies parallel to the wall ofthe heart for ablating a loop like lesion, and a canted disposition, inwhich the ring-like focal region is tilted relative to the wall of theheart to ablate curved-linear lesions.

Conventional methods and apparatus, including the methods and apparatusmentioned above, utilize a continuous mode power profile to ablatecardiac tissue in the treatment of atrial fibrillation. However, withthe conventional methods and apparatus, the collateral tissueimmediately adjacent to the intended ablation target can heat up to atemperature that may result in unwanted necrosis of untargetedcollateral tissue.

This unwanted necrosis of collateral tissue results from excesstemperature elevation in the targeted tissue. Conventional systemsdeliver power in the continuous wave (CW) mode for the entire durationof the ablation cycle which sometimes results in temperature rises inthe targeted tissue in excess of that needed to create necrosis. Heatfrom the target tissue is conducted to nearby collateral tissue andanatomical structures such as the phrenic nerve and esophagus. If theamount of heat energy is sufficiently high, than heat conducted from thetargeted tissue to the collateral tissue results in elevated collateraltissue temperature sufficient to create unwanted necrosis.

Thus, there remains an unmet need for an optimized power deliveryprofile that quickly elevates the targeted tissue to temperaturesresulting in necrosis, then maintains that temperature at a constant ornear constant level for a period of time needed to achieve completetargeted tissue necrosis while, at the same time, ensures that heatconducted to adjacent collateral structures remain insufficient to causeunwanted or untargeted necrosis.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method for applyingenergy within the body of a living subject. The method includesproviding at least one energy emitter adapted to emit energy thatimpinges on a tissue within the body. The method further includesproviding continuous power to the energy emitter for a first durationsufficient to reach a first temperature that is equal to or higher thanthe temperature necessary for necrosis of the tissue. Then, during asecond state, the power to the energy emitter is switched on and off tosubstantially maintain the tissue at a second temperature.

A method according to a related aspect of the invention includes thesteps of providing at least one energy emitter and directing the outputof the energy emitter on a tissue within the body. The energy emitter isconnected to power and the power turned on to emit energy at a firstpower level, for a first duration. The first duration is sufficient toreach a first temperature in the tissue, and the first temperature isequal to or higher than the temperature necessary for necrosis of thetissue. Next, the power is switched to emit energy at a lower powerlevel. The lower power level is sufficient to substantially maintain thetissue at a second temperature.

Another aspect of the present invention provides an apparatus forapplying energy within the body of a living subject. The apparatusincludes an ultrasonic emitter and a housing for the ultrasonic emitter.The housing is adapted to place the ultrasonic emitter so that theoutput from the emitter will be directed to a tissue within the body. Apower supply is connected to the ultrasonic emitter. The power supply isadapted to supply power to the ultrasonic emitter and thereby turn it onfor a first duration sufficient for a tissue to reach a firsttemperature that is equal to or higher than the temperature necessaryfor necrosis of the tissue. Next, the power is cycle between on and offconditions to turn the ultrasonic emitter on and off to substantiallymaintain the tissue at a second temperature.

Apparatus according to further aspect of the invention includes anultrasonic emitter and a housing for the ultrasonic emitter, the housingbeing adapted to place the ultrasonic emitter so that the output fromthe emitter will be directed to a tissue within the body. A power supplyis connected to the ultrasonic emitter. The power supply is adapted tosupply power to the ultrasonic emitter to emit ultrasonic energy at afirst power level, for a first duration, the first duration beingsufficient to reach a first temperature in the tissue, the firsttemperature being equal to or higher than the temperature necessary fornecrosis of the tissue. Next, the ultrasonic emitter is powered to emitat a lower power level, the lower power level being sufficient tosubstantially maintain the tissue at a second temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of apparatus in accordance with oneembodiment of the invention in conjunction with a portion of a heart andpulmonary vein.

FIG. 2 is a diagrammatic sectional view taken along line 2-2 in FIG. 1.

FIG. 3 is a fragmentary diagrammatic view depicting certain geometricalrelationships in the apparatus of FIG. 1.

FIG. 4 is an example of input power profile for the apparatus of FIG. 1.

FIG. 5 is a graph of temperature (at the phrenic nerve) versus timemeasured during animal experiments.

DETAILED DESCRIPTION

FIG. 1 shows one embodiment of ablation device of the invention. Manymore embodiments of ablation device are disclosed in commonly assignedU.S. Pat. No. 6,635,054. Each of these embodiments can be used with theinvention described herein. A portion of a probe structure 10 betweenproximal and distal ends 12 and 14 respectively is omitted in FIG. 1 forclarity of illustration. The probe structure includes a tubular firstcatheter 16 and a tubular second catheter 18 surrounding first catheter16.

First catheter 16 and a cylindrical transducer 20 define a central axis24 adjacent the distal end of the probe structure. First catheter 16 hasa distal tip 26 projecting distally beyond transducer 20. A firstballoon 28, also referred to herein as a “structural balloon,” ismounted to first catheter at the distal end thereof. First balloon 28includes an active wall 32 formed from film which is flexible but whichcan form a substantially noncompliant balloon structure when inflated. Aforward wall 30 may be generally conical or dome-shaped and may projectforwardly from its juncture with active wall 32. For example, forwardwall 30 may be conical, with an included angle of about 120 degrees.Forward wall 30 joins the wall of first catheter 16 at distal tip 26thereof, whereas active wall 32 joins the wall of catheter 16 proximallyof transducer 20. Thus, transducer 20 is disposed inside of firstballoon 28.

The shape of active wall region 32 depicted in FIG. may be that of asurface of revolution about central axis 24 formed by rotation of ageneratrix or curve 38 (FIG. 3) which is a portion of a parabola 40having its principal axis 42 transverse to and desirably perpendicularto central axis 24. Focus 44 of the parabolic generatrix is slightlyforward or distal of forward wall 30 when the balloon is in the inflatedcondition.

A second balloon 50, also referred to herein as the “reflector balloon,”is carried on the distal end of second catheter 18. When both first andsecond balloons 28 and 50, respectively, are in a deflated position,second balloon 50 is collapsed inwardly, toward central axis 24 so thatsecond balloon in deflated condition 50′ closely overlies deflated firstballoon 28′.

In use, the probe structure, with first balloon 28 and second balloon 50deflated, is threaded through the subject's circulatory system.Thereafter, upon inflation of first balloon 28 and second balloon 50,forward wall 30 of first balloon 28 bears on the interior surface of theheart wall at an ostium or opening 74 at which pulmonary vein 72communicates with heart chamber 70.

Transducer 20 is connected to a source 78 of electrical excitationsignals through connector 22. Source 78 is adapted to provide continuousand intermittent electrical excitation. Thus, Source 76 can providecontinuous excitation for a predetermined period of time and then turnthe electrical excitation on and off for a predetermined period of time.For example, after providing continuous excitation for between 5 and 30seconds, source 78 may turn the electrical excitation off for a onesecond and then turn it on for one second and repeat the on-off cyclefor a predetermined period of time. The electrical excitation actuatestransducer 20 to produce ultrasonic waves. The ultrasonic wavespropagate substantially radially outwardly as indicated by arrows 80 inFIGS. 1 and 3. Stated another way, cylindrical transducer 20 producessubstantially cylindrical wave fronts which propagate generally radiallyoutwardly. These waves are reflected by the interface at active region32. Because the interface has a parabolic shape, the waves striking anyregion of the interface will be reflected substantially to focus 44defined by the surface of revolution, i.e., into a substantially annularor ring-like focal region at focus 44. As best seen in FIG. 2, thisring-like focus surrounds central axis 24 and surrounds ostium 74 of thepulmonary vein. This focal region is slightly forward of forward wall 30and hence within the heart tissue, near the surface of the heart wall.For example, the focal region may be disposed at a depth equal to aboutone-half of the thickness of the heart wall as, for example, about 2-4mm from the surface of the wall.

The heart wall tissue located at focus 44 is heated rapidly. The initialCW power delivery Is performed with high power output to quickly createthe initial lesion which creates an absorptive barrier for ultrasoundand therewith protects distal collateral structures. It is believed thatthe lesion will mostly grow towards the source. The temperature of thetissue depends upon several factors including the output power oftransducer 20 and the time for which the tissue is exposed to the outputof transducer 20. Upon the target tissue being exposed to the ultrasonicoutput of transducer 20 for a predetermined time, the target tissuereaches the target temperature, i.e., the temperature that would resultin necrosis_ The target temperature may be in the range 55-80 degreescentigrade, preferably in the range 55-60 degrees centigrade. Thecontinuous excitation is maintained for a first duration sufficient forthe target tissue to reach the target temperature. At the end of thefirst duration, the electrical excitation is turned on and off tomaintain the target tissue at the target temperature. The rapid heatingof the target tissue to the target temperature effectively ablates orkills the tissue at the focal region so that a wall of non-conductivescar tissue forms in the focal region and in neighboring tissue.However, by turning the electrical excitation on and off and therebymaintaining the target tissue at the target temperature, the amount ofneighboring tissue that is killed is minimized. This is in contrast tokeeping the electrical excitation on continuously for the entireduration of time necessary to ablate the target tissue. If theelectrical excitation is kept on for the entire duration of timenecessary to ablate the target tissue, the temperature of the targettissue keeps rising for the entire duration and exceeds the temperaturenecessary for tissue necrosis. This results in necrosis of greateramount of neighboring tissue as compared to when the electricalexcitation is turned on and off during the ablation cycle. For aparticular ablation apparatus using particular transducer, the time ittakes for the target tissue to reach the target temperature may bedetermined via theoretical models or experimentally or by a combinationof these techniques. For a given ablation apparatus, experiments may becarried out wherein the cardiac tissue is ablated and temperature of thetissue at different time measured by known techniques such as use ofthermocouples or imaging. Based upon these experiments, a recommendationfor duration of operation of the ablation apparatus in the continuousmode and duration of operation in the on-off mode would be provided tothe physicians. The process will have to be repeated for an ablationapparatus of a different design.

FIG. 4 shows the input power profile for electrical excitation oftransducer 20. At the beginning of the ablation cycle, transducer 20 issupplied with 100 watt electrical excitation signal. The excitation oftransducer 20 is continuous for approximately 11 seconds. However, thecontinuous excitation mode may range from 5 seconds to 30 seconds or forthe duration necessary for the target tissue to reach the targettemperature. Once the target tissue has reached the target temperaturethe input power is cycled off and on as shown in FIG. 4. The power isoff for roughly 25 percent of the time and the power is on for roughly75 percent of the time. To put it another way, once the target tissuehas reached the target temperature, the power is off for roughly 1second and then it is on for roughly 3 seconds with the on-off cyclecontinuing for the duration of the ablation cycle. The power is turnedoff at the end of the ablation cycle and the temperature of the targettissue drops rapidly thereafter. It should be noted that the poweron-off cycle can be varied, for example, similar results may be obtainedwith the power being off for one second and on for one second followingthe continuous excitation mode. Alternatively, the entire power inputprofile may consist of continuous excitation at one power level and uponreaching of the target temperature switching to continuous excitation ata second lower power level.

FIG. 5 shows plots of temperature measured at a fixed distance from thetarget tissue during animal experiments. A 20 mm HIFU Ablation Cathetermade by Prorhythm, inc., was used. The Ablation catheter was suppliedwith 100 watt electrical excitation signal, and the HIFU output was 32watts acoustic. A 20 mm ablation device kills tissue in the shape of aring of 20 mm diameter. Several embodiments of ablation devices that cancause necrosis of tissue in shape of a ring are disclosed in U.S. Pat.No. 6,635,054 and U.S. Patent Application Publication No. US2004/0176757. As seen in FIG. 5, plot A, the temperature at the fixeddistance from the target tissue keeps rising for the entire duration ofthe ablation cycle when the electrical excitation is kept on for theentire duration of the ablation cycle. On the other hand, as seen inFIG. 5, plot B, when the electrical excitation is turned on and offduring a portion of the ablation cycle, the temperature at the fixeddistance from the target tissue rises during the continuous excitationmode and then remains substantially constant at that level during theon-off mode. This results in reduction or elimination of collateraltissue damage.

Some of the ultrasonic energy is absorbed between the surface of thewall and the focal region, and at locations deeper within the wall thanthe focal region. To provide a complete conduction block, tissue shouldbe ablated through the entire thickness of the wall, so as to form atransmural lesion. With a transducer capable of emitting about 15 Wattsof acoustic energy, an ablated region extending entirely through theheart wall can be formed within a few minutes of actuation. Higher powerlevels as, for example, above 30 Watts of acoustic energy and desirablyabout 45 Watts are preferred because such power levels will provideshorter lesion formation time (under one minute). Because the sonicenergy is directed simultaneously into the entire loop-like pathsurrounding the pulmonary vein, the PV isolation can be performedideally without repositioning the probe. However, several applicationsmay be required due to non circular, irregular anatomy.

The positioning of the ablation device within the heart desirablyincludes selectively controlling the disposition of theforward-to-rearward axis 24 of the device relative to the patient'sheart. That is, the position of the forward-to-rearward axis desirablycan be controlled by the physician to at least some degree. To that end,the assembly can be provided with one or more devices for selectivelybending the ablation device. Various embodiments of the ablation devicethat lend themselves to allow disposition of the ablation device to beselectively controlled are disclosed in commonly assigned PatentApplication No. US 2004/0176757. Each of these embodiments may be usedin conjunction with the input power profile disclosed herein. Althoughthe invention has been described with the aid of an ablation deviceusing HIFU, any form of output power for ablating the tissue may be usedin the on-off mode as described herein to realize the benefit of theinvention. Non limiting examples of the other forms of output power areRF and heat.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

INDUSTRIAL APPLICABILITY

This application relates to the medical device industry.

What is claimed:
 1. An apparatus for applying energy intraluminally to atarget tissue, the apparatus comprising: a catheter having a distalregion adapted for placement adjacent to the target tissue; anultrasound transducer disposed at the distal region of the catheter; aballoon disposed at the distal region of the catheter, wherein theultrasound transducer is disposed within the balloon; an energy emissionsource configured to provide electrical energy to the ultrasoundtransducer, the energy emission source configured to cause theultrasound transducer to emit energy at an initial level for a firsttime duration sufficient to necrose at least a portion of the targettissue so as to create an initial lesion that forms a protective barrierto obstruct subsequently emitted energy traveling toward collateralanatomical structures distal to the initial lesion, the energy emissionsource further configured to, upon completion of the first timeduration, cause the ultrasound transducer to emit modulated energy bycycling between a first power level and a second power level to deliverenergy to the protective barrier and target tissue for a second timeduration to completely necrose the entire volume of target tissue,wherein the protective barrier reduces damage to untargeted tissuedistal to the target tissue, and wherein the energy emission sourcecauses the ultrasound transducer to emit modulated energy based on anempirical determination without monitoring temperature.
 2. The apparatusof claim 1, further comprising a reflector at the distal region of thecatheter, the reflector having an active region to redirect energy fromthe ultrasound transducer, wherein the active region defines a focalregion comprising a volume of the target tissue.
 3. The apparatus ofclaim 2, wherein the reflector is positioned within the balloon.
 4. Theapparatus of claim 2, wherein the reflector comprises a collapsibleballoon positioned within the balloon.
 5. The apparatus of claim 2,wherein the reflector is parabolic in shape and causes the active regionto define a circular focal region.
 6. The apparatus of claim 1, whereinthe distal region of the catheter is adapted for placement within ablood vessel.
 7. The apparatus of claim 1, wherein the first power levelis greater than the second power level.
 8. The apparatus of claim 1,wherein the initial power level is greater than the first power level.9. The apparatus of claim 1, wherein no energy is delivered at thesecond power level.
 10. An apparatus for applying energy intraluminallyto a target tissue, the apparatus comprising: a catheter having a distalregion adapted for placement adjacent to the target tissue; and anultrasound transducer disposed at the distal region of the catheter, theultrasound transducer configured to emit energy at an initial level fora first time duration sufficient to necrose at least a portion of thetarget tissue so as to create an initial lesion that forms a protectivebarrier to obstruct subsequently emitted energy traveling towardcollateral anatomical structures distal to the initial lesion, theultrasound transducer further configured to, upon completion of thefirst time duration, emit modulated energy by cycling between a firstpower level and a second power level to deliver energy to the protectivebarrier and the target tissue for a second time duration to necrose thetarget tissue.
 11. The apparatus of claim 10, wherein the target tissuecomprises a transmural area of an organ or a blood vessel.
 12. Theapparatus of claim 10, wherein the distal region of the catheter isadapted for placement within a blood vessel.
 13. The apparatus of claim10, wherein the initial power level is greater than the first powerlevel.
 14. The apparatus of claim 10, wherein no energy is delivered atthe second power level.
 15. The apparatus of claim 10, wherein theultrasound transducer is configured to emit modulated energy based on anempirical determination without monitoring temperature.
 16. Theapparatus of claim 10, wherein the protective barrier reduces damage tountargeted tissue distal to the target tissue during the second timeduration.
 17. The apparatus of claim 10, further comprising an energyemission source configured to provide electrical energy to theultrasound transducer.
 18. The apparatus of claim 10, further comprisinga balloon disposed at the distal region of the catheter, wherein theultrasound transducer is disposed within the balloon.
 19. The apparatusof claim 10, further comprising a reflector at the distal region of thecatheter, the reflector having an active region to redirect energy fromthe ultrasound transducer, wherein the active region defines a focalregion comprising a volume of the target tissue.
 20. The apparatus ofclaim 19, wherein the reflector is parabolic in shape and causes theactive region to define a circular focal region.